Gravitational Lenses as High-Resolution Telescopes

The inner regions of active galaxies host the most extreme and energetic phenomena in the universe including, relativistic jets, supermassive black hole binaries, and recoiling supermassive black holes. However, many of these sources cannot be resolved with direct observations. I review how strong gravitational lensing can be used to elucidate the structures of these sources from radio frequencies up to very high energy gamma rays. The deep gravitational potentials surrounding galaxies act as natural gravitational lenses. These gravitational lenses split background sources into multiple images, each with a gravitationally-induced time delay. These time delays and positions of lensed images depend on the source location, and thus, can be used to infer the spatial origins of the emission. For example, using gravitationally-induced time delays improves angular resolution of modern gamma-ray instruments by six orders of magnitude, and provides evidence that gamma-ray outbursts can be produced at even thousands of light years from a supermassive black hole, and that the compact radio emission does not always trace the position of the supermassive black hole. These findings provide unique physical information about the central structure of active galaxies, force us to revise our models of operating particle acceleration mechanisms, and challenge our assumptions about the origin of compact radio emission. Future surveys, including LSST, SKA, and Euclid, will provide observations for hundreds of thousands of gravitationally lensed sources, which will allow us to apply strong gravitational lensing to study the multi-wavelength structure for large ensembles of sources. This large ensemble of gravitationally lensed active galaxies will allow us to elucidate the physical origins of multi-wavelength emissions, their connections to supermassive black holes, and their cosmic evolution.Comment: Invited (Accepted) review for Physics Report

Galaxies as High-Resolution Telescopes

Recent observations show a population of active galaxies with milliarcseconds offsets between optical and radio emission. Such offsets can be an indication of extreme phenomena associated with supermassive black holes including relativistic jets, binary supermassive black holes, or even recoiling supermassive black holes. However, the multi-wavelength structure of active galaxies at a few milliarcseconds cannot be fathomed with direct observations. We propose using strong gravitational lensing to elucidate the multi-wavelength structure of sources. When sources are located close to the caustic of lensing galaxy, even small offset in the position of the sources results in a drastic difference in the position and magnification of mirage images. We show that the angular offset in the position of the sources can be amplified more than 50 times in the observed position of mirage images. We find that at least 8% of the observed gravitationally lensed quasars will be in the caustic configuration. The synergy between SKA and Euclid will provide an ideal set of observations for thousands of gravitationally lensed sources in the caustic configuration, which will allow us to elucidate the multi-wavelength structure for a large ensemble of sources, and study the physical origin of radio emissions, their connection to supermassive black holes, and their cosmic evolution.Comment: Accepted for publication in Ap

PKS 1510-089: a rare example of a flat spectrum radio quasar with a very high-energy emission

The blazar PKS 1510-089 is an example of flat spectrum radio quasars. High-energy emissions from this class of objects are believed to have been produced by inverse Compton radiation with seed photons originating from the broad line region. In such a paradigm, a lack of very high-energy emissions is expected because of the Klein-Nishina effect and strong absorption in the broad line region. Recent detection of at least three such blazars by Cherenkov telescopes has forced a revision of our understanding of these objects. We have aimed to model the observed spectral energy distribution of PKS 1510-089 from the high-energy flares in March 2009, during which very high-energy emission were also detected by H.E.S.S. We have applied the single-zone internal shock scenario to reproduce the multiwavelength spectrum of PKS~1510-089. We have followed the evolution of the electrons as they propagate along the jet and emit synchrotron and inverse Compton radiation. We have considered two sources of external photons: the dusty torus and the broad line region. We have also examined the effects of the gamma-gamma absorption of the high-energy photons both in the AGN environment (the broad line region and the dusty torus), as well as while traveling over cosmological distances: the extragalactic background light. We have successfully modeled the observed spectrum of PKS 1510-089. In our model, the highest energy emission is the result of the Comptonization of the infrared photons from the dusty torus, thus avoiding Klein-Nishina regime, while the bulk of the emissions in the GeV range may still be dominated by the Comptonization of radiation coming from the broad line region.Comment: Accepted for publication in A&

Searching for VHE gamma-ray emission associated with IceCube neutrino alerts using FACT, H.E.S.S., MAGIC, and VERITAS

The realtime follow-up of neutrino events is a promising approach to search for astrophysical neutrino sources. It has so far provided compelling evidence for a neutrino point source: the flaring gamma-ray blazar TXS 0506+056 observed in coincidence with the high-energy neutrino IceCube-170922A detected by IceCube. The detection of very-high-energy gamma rays (VHE, E > 100 GeV) from this source helped establish the coincidence and constrained the modeling of the blazar emission at the time of the IceCube event. The four major imaging atmospheric Cherenkov telescope arrays (IACTs) - FACT, H.E.S.S., MAGIC, and VERITAS - operate an active follow-up program of target-of-opportunity observations of neutrino alerts sent by IceCube. This program has two main components. One are the observations of known gamma-ray sources around which a cluster of candidate neutrino events has been identified by IceCube (Gamma-ray Follow-Up, GFU). Second one is the follow-up of single high-energy neutrino candidate events of potential astrophysical origin such as IceCube-170922A. GFU has been recently upgraded by IceCube in collaboration with the IACT groups. We present here recent results from the IACT follow-up programs of IceCube neutrino alerts and a description of the upgraded IceCube GFU system

Detection techniques for the H.E.S.S. II telescope, data modeling of gravitational lensing and emission of blazars in HE-VHE astronomy

La présente thèse porte sur quatre aspects différents de l astronomie des hautes énergies.La première partie de ma thèse est dédiée à un développement instrumental pour les télescopes Cherenkov imageurs, le système de déclenchement de niveau 2 du télescope de 28 mètres du réseau H.E.S.S. (High Energy Stereoscopic System). Mon travail s est focalisé sur l invention d algorithmes et les simulations Monte-Carlo du système de déclenchement, ainsi que la comparaison de la reconstruction au niveau de la carte de déclenchement à la reconstruction hors-ligne (Moudden,Barnacka, Glicenstein et al. 2011a; Moudden, Venault, Barnacka et al. 2011b). Je décris le système et j évalue ses performances.Le réseau H.E.S.S. a observé le blazar PKS1510-089. La deuxième partie de ma thèse traite de l analyse des données et la modélisation de l émission large bande de ce blazar. C est un exemple de quasar radio à spectre plat (FSRQ), pour lequel il n est attendu aucune émission aux très hautes énergiesJ ai modélisé les données observées pendant un "flare de PKS1510-089. Ce modèle est basé sur un scénario de choc interne à une zone.Le troisième chapitre de ma thèse est une étude d un autre phénomène affectant potentiellement les blazars observés par FERMI-LAT: l effet de lentille gravitationnelle fort. Cette partie de ma thèse montre le premier indice de présence d un effet de lentille gravitationnelle dans le domaine des photons de haute énergie. Cet indice provient de l observation d un écho dans la courbe de lumière du blazar distant PKS1830-211, qui est une lentille gravitationnelle connue. Les méthodes d estimation des retards temporels dans les systèmes de lentille gravitationnelles reposent sur la corrélation croisée des courbes de lumière individuelles. Dans l analyse présentée dans cette thèse, j ai utilisé des photons de 300 MeV à 30 GeV détectés par l instrument FERMI-LAT. L instrument FERMI-LAT ne peut pas séparer spatialement les images des lentilles gravitationnelles fortes connues. La courbe de lumière observée est donc la superposition des courbes de lumière des images individuelles. Les données du FERMI-LAT ont l avantage d être des séries temporelles régulièrement espacées,avec un bruit de photons très bas. Cela permet d utiliser directement les méthodes de transformées de Fourier. Un retard temporel entre les images compactes de PKS1830-211 a été recherché par deux méthodes : une méthode d auto-corrélation et la méthode du double spectre . La méthode du double spectre fournit un signal de 27 +- 0.6jours (statistique) avec une significativité de 4.2 . Ce résultat est compatible avec ceux de Lovell et al (1998) et Wiklind et Combes (2001).La dernière partie de ma thèse est consacrée à un effet de lentille différent, le femtolensing . La recherche d effets de femtolensing a été utlisée pour obtenir des limites sur l abondance de trous noirs primordiaux. Celle-ci a été contrainte de manière significative dans un large domaine de masses. Les limites les moins contraignantes ont été établies pour les objets de faible masse, pour lesquels la détection représente un défi expérimental. J ai utilisé les sursauts gamma de redshift connus d etectés par le Fermi Gamma Ray Burst Monitor (GBM) pour rechercher d éventuels effets de femtolensing produits par des objets compacts sur la ligne de visée. L absence de ces effets de femtolensing montre que des trous noirs primordiaux de masse comprises entre 5.10^17 et 10^20 g ne constituent pas une fraction importante de la matière noire. J ai effectué mes études de thèse en co-tutelle entre le Centre Astronomique Nicolaus Copernicus de l académie des sciences polonaise, à Varsovie et l Institut de Recherches sur les Lois fondamentales de l Univers du CNRS à Saclay, en France.This thesis presents the study of four aspects of high energy astronomy.The first part of my thesis is dedicated to an aspect of instrument development for imaging atmospheric Cherenkov telescopes, namely the Level 2 trigger system of the High Energy Stereoscopic System (H.E.S.S.). My work on the project focused on the algorithm development and the Monte Carlo simulations of the trigger system and overall instrument (Moudden, Barnacka, Glicenstein et al. 2011a; Moudden, Venault, Barnacka et al. 2011b). The hardware implementation of the system is described andits expected performances are then evaluated. The H.E.S.S. array has been used to observe the blazar PKS 1510-089.The second part of my thesis deals with the data analysis and modeling of broad-band emission of this particular blazar. In part II of my thesis, I am presenting the analysis of the H.E.S.S. data: the light curve and spectrum of PKS 1510-089, together with the FERMI data and a collection of multi-wavelength data obtained with various instruments. I am presenting the model of PKS 1510-089 observations carried out during a flare recorded by H.E.S.S.. The model is based on a single zone internal shock scenario.The third part of my thesis deals with blazars observed by the FERMI-LAT, but from the point of view of other phenomena: a strong gravitational lensing. This part of my thesis shows the first evidence for gravitational lensing phenomena in high energy gamma-rays. This evidence comes from the observation of a gravitational lens system induced echo in the light curve of the distant blazar PKS 1830-211. Traditionalmethods for the estimation of time delays in gravitational lensing systems rely on the cross-correlation of the light curves from individual images. In my thesis, I used 300 MeV-30 GeV photons detected by the Fermi-LAT instrument. The FERMI-LAT instrument cannot separate the images of known lenses. The observed light curve is thus the superposition of individual image light curves. The FERMI-LAT instrument has the advantage of providing long, evenly spaced, time series with very low photonnoise. This allows to use directly Fourier transform methods. A time delay between the two compact images of PKS 1830-211 has been searchedfor both by the autocorrelation method and a new method: the double power spectrum . The double power spectrum shows a 4.2 evidence for a time delay of 27.1+-0.6 days (Barnacka et al. 2011), consistent with the results from Lovell et al. (1998) and Wiklind & Combes (2001).The last part of my thesis concentrates on another lensing phenomena called femtolensing . The search for femtolensing effects has been used to derive limits on the primordial black holes abundance. The abundance of primordial black holes is currently significantly constrained in a wide range of masses. The weakest limits are established for the small mass objects, where the small intensity of the associated physical phenomenon provides a challenge for current experiments. I have usedgamma-ray bursts with known redshifts detected by the FERMI Gamma-ray Burst Monitor (GBM) to search for the femtolensing effects caused by compact objects. The lack of femtolensing detection in the GBM data provides new evidence that primordial black holes in the mass range 5 . 10^17 10^20 g do not constitute a major fraction of dark matter (Barnacka et al. 2012). My Ph.D. studies have been carried out jointly between the Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences, in Warsaw in Poland and the IRFU institute of the Commissariat à l énergie atomique et aux énergies alternatives(CEA) Saclay in France.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF